1. Field of the Invention
The invention relates to an improved piezoelectric actuator for actuating a mechanical component such as a valve or the like.
2. Description of the Prior Art
It is generally known that by using the so-called piezoelectric effect, a piezoelectric element can be constructed from a material having a suitable crystalline structure. When an external electrical voltage is applied, a mechanical reaction of the piezoelectric element ensues, which as a function of the crystalline structure and the regions of contact with the electrical voltage exerts a pressure or tension in a predeterminable direction.
From German Patent Disclosure DE 196 50 900, for instance, a piezoelectric actuator is known which is suitable particularly for actuating control valves or injection valves in motor vehicles. To that end, it comprises layers, stacked on one another in the manner of a laminate, of piezoelectric material with metal or electrically conductive layers serving as electrodes located between them. The actuator body thus formed, on each of whose end faces a respective head plate toward the valve, with an axially protruding valve tappet, and an opposed foot plate are attached, is inserted into a retaining bore of a valve housing. On the face end, the actuator body is prestressed in the axial direction, between the head plate and the foot plate, by a spring element.
As mentioned at the outset, such piezoelectric multilayer actuators, when they are subjected to a pulsating electrical voltage on their electrode layers, execute similarly pulsating strokes, changing the spacing between their two face ends. The piezoelectric multilayer actuator must be supported in the steel housing of the injection valve in such a way that short circuits cannot occur, which means that a centered installation with a defined spacing from the outer wall must be assured, and there must be no risk of short circuits between the actuator body and the outer wall.
In operation of such piezoelectric multilayer actuators, heat occurs in the actuator body, and if destruction and impairment of the function of the multilayer actuator is to be avoided, this heat must be dissipated to the outside, or in this case to the valve housing of the injection valve or control valve. In the known multilayer actuator mentioned above, the spring sleeve absorbs the heat generated by the actuator body and carries it to the outside and simultaneously assures the centering, required for installation, of the actuator body in the valve housing. It is intrinsically known from German Patent DE 197 15 488 C1 that an actuator body be surrounded with a polymer envelope, but this envelope leaves an air gap open between itself and the inner wall of the valve housing. The result is the disadvantage that the dissipation of heat through the air gap to the valve housing is made more difficult.
In the piezoelectric actuator of the invention, it is assumed that there is a piezoelectric element which is suitable for subjecting an actuating element to a tensile or compressive stress and which is provided with a foot part to which the piezoelectric element is secured and by way of which the piezoelectric element is kept centered in a housing with mechanical prestressing. Advantageously, a sleeve is secured to the foot part and surrounds the piezoelectric element in such as way as to stabilize it mechanically, at least in partial regions that are electrically insulated from the piezoelectric element.
It is especially advantageous if a heat-conducting elastomer is placed between the sleeve and the piezoelectric element, and the sleeve comprises a heat-conducting material that is deformable within predetermined limits. This component group containing the piezoelectric element and the sleeve on the foot part can be press-fitted in a simple way into a retaining bore of the housing, resulting in good thermal conduction from the piezoelectric multilayer actuator, for instance to a valve housing. The foot part can also have an encompassing groove, into which a likewise encompassing groove can be latched on the inside diameter of the sleeve.
With the sleeve, the piezoelectric actuator has a surface that is not vulnerable, for instance to chemical factors, and it can be installed simply, and an actuator module of this kind is also protected against harmful impairments during shipping and in assembly. The piezoelectric actuator cast in the sleeve is both centered and secured against rotation by the elastomer, since the flow of force is not only via the foot part but additionally via the elastomer to the sleeve that is also joined to the foot part.
Compared to a piezoelectric actuator without a sleeve and sheathed with only a heat-conducting elastomer, the production process is also more economical, since the requisite care taken of the casting molds is unnecessary. Assembling the piezoelectric actuator of the invention is also simpler, because injuries to the sheathing and the attendant output of heat-conductive particles, with consequent soiling of the retaining bore, cannot occur, as they can in a piezoelectric actuator that is sheathed only with elastomer.
The sheath of the invention, which is as a rule thin-walled and which comprises the heat-conducting and deformable material, such as copper, steel or a plastic, is solidly joined to the foot part of the piezoelectric element, and to improve the centering, it can have a slightly increasing outer diameter toward the top. A metal sleeve, for instance, can be produced economically in the form of a rolled component of stamped sheet metal. A plastic sleeve, for instance in the form of an injection-molded part, is then allowed to touch the piezoelectric element and can thus lend the actuator module greater stability even without elastomer sheathing.
In a further embodiment, it is advantageous if by means of centering rings additionally placed in the sleeve and preferably having a hollow profile, the piezoelectric element can be centered even better in the sleeve, while at the same time space is created for the compensation of thermal and press-fitting expansion.
By means of an oblique shoulder in the retaining bore, a calibrating press-fitting of the sleeve can also be made possible in a simple way, thus assuring a close contact of the sleeve with the retaining bore, as a result of which very good heat conduction radially outward takes place.
In another embodiment of the piezoelectric actuator of the invention, the sleeve is perforated or slotted, so that the heat-conducting elastomer can flow through this sleeve and directly produces the heat conduction between the piezoelectric element and the housing having the retaining bore. The casting of the piezoelectric element can be done either in the housing or in a re-usable or lost-type mold.
These and other characteristics of preferred refinements of the invention can be learned from the description and the drawings; the individual characteristics may each individually or in the form of subcombinations with each other be realized in the embodiment of the invention and in other fields and can represent both advantageous and intrinsically patentable embodiments for which patent protection is here claimed.